New Clue May Solve Solar Mystery

Below:

Next story in Science

A new look at the sun using views from two different spacecraft
has put a big dent in the nagging mystery of why the sun's
atmosphere, or corona, is millions of degrees hotter than its
surface.

The key, it turns out, are called spicules, which vent hot gases,
called plasma, from the sun's surface into the corona.

In years past, spicules had been judged too cool to be a source
of the high temperature plasma that, among other things,
generates the sun's ultraviolet light. A few years ago, a new
kind of faster, shorter-lived spicule was discovered. These Type
II spicules shoot up at 100 to 150 kilometers per second (62 to
93 miles per second), then disappear.

That behavior made them rather suspicious, but until now, there
was no way to check whether they were at least part of the
coronal heating puzzle.

"It's a little jet, then it takes off," said solar physicist
Scott McIntosh of the National Center for Atmospheric Research's
High Altitude Observatory. Those jets have now been measured
sometimes exceeding a million degrees. "What we basically find is
that the connection is the heated blobs of plasma. It's kind of a
missing link that we've been looking for since the 1960s."

The discovery was made by combining data from the new Atmospheric
Imaging Assembly on NASA's recently launched Solar Dynamics
Observatory with data from NASA's Focal Plane Package for the
Solar Optical Telescope on the Japanese Hinode satellite. The
combined observations gave the researchers enough resolution to
reveal a one-to-one connection between the heated plasma and
spicules jetting out into the corona.

"It's not solving the problem but adding a little bit of
clarity," said McIntosh, a coauthor of a paper reporting on the
discovery in the latest issue of the journal Science.

This is not an isolated event, said McIntosh. The Type II
spicules appear to be doing it everywhere, he said.

"This phenomenon is truly ubiquitous and populates the solar
wind," said McIntosh, referring to the wind of hot particles that
pours out of the sun and out into space.

That said, there is still a ways to go to show that there are
enough spicules at work.

"It is very nice work, but it is absolutely not the final story
on the origin of hot coronal plasma," said NASA solar physicist
James Klimchuk. "Based on some simple calculations I have done,
spicules account for only a small fraction of the hot plasma."

Most of the heat in the corona, said Klimchuk, is apparently
still released by the snapping of stressed magnetic fields in the
corona.

"More work needs to be done before we know what role spicules
play on a global scale," said Klimchuk. "One thing seems certain,
however: (Spicules) are very important within the particular
magnetic flux tubes in which they occur. The new observations are
very exciting, but the jury is still out on the importance of
spicules in the big scheme of things."